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A&A
Volume 695, March 2025
Article Number A182
Number of page(s) 10
Section Stellar atmospheres
DOI https://doi.org/10.1051/0004-6361/202452387
Published online 18 March 2025

© The Authors 2025

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1 Introduction

With lifespans of tens of billions of years (Adams & Laughlin 1997), M dwarfs are highly prevalent in the stellar population of the Galaxy, making up approximately 70% of it (Henry et al. 1994; Reid et al. 1995; Reylé et al. 2021). Thanks to their small size and low luminosity, the habitable zone of these cool stars is much closer than in their solar-like counterparts. This facilitates the detection of potentially habitable Earth-like exoplanets orbiting them, making M dwarfs major targets in the search for exoplanets. On the other hand, the nearby habitable zone around M dwarfs makes exoplanets more exposed to energetic events linked to stellar activity (Tilley et al. 2019; Günther et al. 2020; Chen et al. 2021), such as flares or coronal mass ejections. Stellar flares are sudden releases of magnetic energy caused by magnetic reconnection events in the stellar atmosphere; they are accompanied by bursts of isotropic electromagnetic radiation (Benz & Güdel 2010) and are frequent phenomena in low-mass stars. Flares can release energies of up to ∼ 1037erg (Davenport 2016) in time spans ranging from minutes to several hours, and their spectrum is often modelled as a blackbody with a temperature in the range 9000–10 000 K (Davenport et al. 2020; Günther et al. 2020).

A large fraction of M dwarfs are magnetically active, with a chromospheric activity often diagnosed using Hα or Ca II H and K line emission (Cincunegui et al. 2007; Ibañez Bustos et al. 2023). The relation between the chromospheric emission in these lines is not straightforward; it exhibits a complex behaviour when studied over a large sample of M dwarfs because the activity level in these two regions is not always correlated (Meunier et al. 2024). Moreover, flare emission in the optical domain is also known to occur in these lines (Heinzel et al. 1994; Kowalski et al. 2013). In optical spectra of low-mass stars, strong emission lines have been detected serendipitously, both as transient flare events (Martín & Ardila 2001; Liebert et al. 2003; Schmidt et al. 2007) and as steady features (Mould et al. 1994; Martín et al. 1999b; Burgasser et al. 2011).

Recent multi-filter photometric surveys, such as the Javalambre Photometric Local Universe Survey (J-PLUS; Cenarro et al. 2019) or the upcoming Javalambre Physics of the Accelerated Universe Astrophysical Survey (J-PAS; Benitez et al. 2014), with unique systems of 12 and 56 optical filters, respectively, may enable new ways to systematically detect strong emission lines in low-mass stars (Mas-Buitrago et al. 2022). The spectral energy distributions (SEDs) provided by these surveys are suitable for identifying excess emission in narrow-band filters located at specific spectral features, such as the Hα or Ca II H and K lines, which could be caused by stellar flaring activity. Although the short exposure time characteristic of multi-filter photometric surveys hinders the detection of flare events, the large number of stars that can be analysed simultaneously in this way makes it feasible.

High-precision, high-cadence photometric light curves (LCs) provided by surveys such as Kepler (Borucki et al. 2010), the Kepler extended mission (K2; Howell et al. 2014), and the Transiting Exoplanet Survey Satellite (TESS; Ricker et al. 2015) are a powerful asset for understanding and studying the stellar activity of low-mass stars and brown dwarfs (e.g. Martín et al. 2013; Doyle et al. 2019, 2022; Kumbhakar et al. 2023). TESS is a NASA mission launched in April 2018 with the primary objective of searching for transiting exoplanets around bright, nearby stars. Operating at optical wavelengths, the high-cadence photometric data provided by TESS capture insightful information about the magnetic activity of target stars, mainly in the form of periodic variability due to co-rotating star spots or as sudden brightness outbursts caused by stellar flares.

We present a spectroscopic follow-up of the sample of M dwarfs with strong Hα and Ca II H and K emission identified by Mas-Buitrago et al. (2022) using J-PLUS photometry. Using TESS LCs, we performed a comprehensive study of the stellar activity of two active M dwarfs found in our sample, including a detailed analysis of their flaring activity and rotation period, as well as an estimation of their age. Moreover, we discuss the possible implications of the observed stellar activity on the evolution and habitability of exoplanets orbiting low-mass stars. In Sect. 2 we describe the spectroscopic observations collected for our sample. The reduced spectra of our targets and the analysis performed using TESS LCs are discussed in Sect. 3. Section 4 presents a discussion of the impact of the magnetic activity of M dwarfs on planetary habitability. Finally, the main conclusions of this work are summarised in Sect. 5.

Table 1

Targets selected for spectroscopic observation.

2 Observations

2.1 Sample selection

The sample studied in this work is the result of the search for strong emission lines performed in our previous work (Mas-Buitrago et al. 2022), using multi-filter optical photometry from J-PLUS (Cenarro et al. 2019). For this, we developed a Python algorithm capable of detecting excess in the J-PLUS filters corresponding to the Hα (J 0660) and Ca II H and K (J0395) emission lines. Following this approach, we identified eight M dwarfs with emission excess in these filters (four of them in each of the filters and none showing both excesses simultaneously). In the end, one of these objects was discarded for spectroscopic follow-up because it was not bright enough, resulting in a final sample of seven M dwarfs. Table 1 lists the selected targets.

The J-PLUS SED of each target star is provided in Appendix A. The excess emission in the J0395 filter is evident for J-PLUS0114, J-PLUS0744, J-PLUS0807, and J-PLUS0903. On the other hand, the SEDs of J-PLUS0226, J-PLUS0708, and J-PLUS0914 show strong emission in the J0660 filter. We attribute this behaviour to the fact that the star experiences flaring activity during the corresponding J-PLUS observing block, in which all filters are observed sequentially. The strategy for each J-PLUS observing block is to obtain, for the same pointing, three consecutive exposures per filter, with a total exposure time of approximately one hour (Cenarro et al. 2019). Flaring phenomena during the exposures for the filters of interest would explain the SED behaviour found. Given the low probability of observing a flare during the exposures for the filters of interest, it is easier to detect the less energetic and shorter-lived flares, which are more frequent and last a few minutes, as we confirm in Sect. 3.2.1.

The estimated effective temperatures for these objects, obtained with the tool VOSA1 (Bayo et al. 2008), classify them as mid-M dwarfs except for one, namely LP 310-34, which has a Teff = 2500 K. As mentioned in Mas-Buitrago et al. (2022), we carried out a spectroscopic follow-up for LP 310-34 that confirmed it as a late M dwarf (dM8) with Hα in emission (Schmidt et al. 2007).

2.2 Observational details

We collected low-resolution optical spectra of our seven targets with the Alhambra Faint Object Spectrograph and Camera (ALFOSC) mounting on the 2.56−m Nordic Optical Telescope (NOT) with proposal number 66-208 (PI ELM). Also, we observed two bright targets (J-PLUS DR2 J0807+32 and J-PLUS DR2 J0903+34 in Table 1) with the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) mounting on the 10.4 m Gran Telescopio Canarias (GTC), at the Roque de los Muchachos Observatory on the island of La Palma, Spain, with programme GTCMULTIPLE2I-22B (PI ELM).

ALFOSC is equipped with a Teledyne e2v CCD231-42-gF61 back illuminated, deep depletion, astro multi-2 detector. The detector dimension is 2048 × 2064 pixels with a scale of 0.2138 arcsec/pix. The NOT/ALFOSC observation was executed under visitor mode on the nights of January 26–27, 2023 (observers PMB and JYZ). We used a 1.0 arcsec slit, and #4 grism, which provide a wavelength range from 3200 Å to 9600 Å with a resolution power R ≈ 360.

OSIRIS is a commonly used instrument of GTC. It covers the wavelength range 3650–10050 Å and has an effective field of view of 7.5 × 6.0 arcmin. OSIRIS has two Marconi CCD44-82 (2048 × 4096 pixels) detectors with a gap in between. The 2 × 2 binned pixel size is 0.254 arcsec/pix. In the mode of longslit spectroscopy, the object is centred on the slit at the coordinate X = 250 of CCD2. The GTC/OSIRIS observation was executed under service mode. We required a maximum seeing of 1.2 arcsec, a cloud free sky, and a grey moon phase, using the R1000B grism and a 1.2 arcsec slit under the parallactic angle. This configuration yields a wavelength coverage from 3600 Å to 7900 Å with a resolution power R ≈ 500. The record of observations is provided in Appendix B.

thumbnail Fig. 1

Co-added spectra observed with NOT/ALFOSC (left panel) and GTC/OSIRIS (right panel), sorted from top to bottom by the derived PC3 index (see Table 2). The grey shaded bands in the left panel show the spectral regions used to derive the PC3 index.

2.3 Data reduction

We reduced both the GTC/OSIRIS and NOT/ALFOSC data using v 1.12 of PypeIt (Prochaska et al. 2020a,b), a community-developed open-source Python package for semi-automated reduction of spectroscopical data in astronomy. PypeIt supports a long list of spectrographs and provides the code infrastructure to automatically process the image, identify the slit in a given detector, extract the object spectra, and perform wavelength calibration. We observed the standard stars HD 19445 and Feige 110 for flux calibration of NOT/ALFOSC and GTC/OSIRIS data, respectively.

3 Results and discussion

3.1 Reduced spectra

Figure 1 shows the co-added spectra for the observed targets. We note that no intense steady line emission is observed in the spectra (for examples of strong line emission in low-resolution spectra, see Fig. 1 in Burgasser et al. 2011 or Figs. 10 and 11 in Schmidt et al. 2007), confirming that the excess emission detected in the J-PLUS photometry is not steady and is indeed caused by transient flaring activity. Moreover, objects with excess emission in the J-PLUS Ca II H and K filter show no apparent differences in their spectral features compared to objects with excess emission in the J-PLUS Hα filter (see Table 1), suggesting that the flaring activity detected in Ca II H and K is not particular to a specific type of star. Hence, it follows that common M dwarfs experience two types of flares, those already well known in Hα and those in Ca II H and K revealed in this work.

Several spectroscopic indices have been explored for the spectral classification of M dwarfs (Lépine et al. 2003) and, in particular, for late-M dwarfs using low-resolution optical spectra (Kirkpatrick et al. 1995; Martin et al. 1996; Martín et al. 1999a). To derive spectral types for our sample, we measured the PC3 index (Martín et al. 1999a), which is a reliable indicator of spectral type in the [M2.5, L1] range and has been used consistently in the literature (Crifo et al. 2005; Martin et al. 2006, 2010; Phan-Bao & Bessell 2006; Reylé et al. 2006; Phan-Bao et al. 2008). The PC3 index is a pseudo-continuum spectral ratio between the 8230–8270 Å (numerator) and 7540–7580 Å (denominator) intervals, which can be used to derive spectral types between M 2.5 and L1 following the calibration presented by Martín et al. (1999a): SpT=−6.685+11.715×(PC3)−2.024×(PC3)2.$\mathrm{SpT}=-6.685+11.715 \times(\mathrm{PC} 3)-2.024 \times(\mathrm{PC} 3)^{2}.$(1)

Table 2 lists the PC3 index and the adopted spectral type, with an uncertainty of ±0.5 sub-classes, for our targets. The classification obtained for J-PLUS0807 is consistent with that provided in Schmidt et al. (2007), who derived a spectral type of dM8, with an uncertainty of ±0.5 sub-classes, by visual comparison of the spectra to spectral standards. These results confirm the rest of our sample, still spectroscopically unclassified in the literature, as mid-M dwarfs.

The obtained spectra confirm both J-PLUS0807 and JPLUS0903 as active M dwarfs with Ca II H and K and Hα in emission, while the rest of the targets show no signs of activity. Figure 2 shows a close-up view of the spectral region of interest for these stars, with prominent Ca II H and K, Hδ, Hγ, Hβ and Hα emission lines. We quantified the Hα emission using the specutils2 (Earl et al. 2022) Python package, obtaining an Hα equivalent width of −16.80 Å and −5.90 Å for the co-added NOT/ALFOSC spectra, and of −18.36 Å and −6.08 Å for the coadded GTC/OSIRIS spectra of J-PLUS0807 and J-PLUS0903, respectively. These results correspond to levels of Hα emission that are not uncommon among this type of stars (Schmidt et al. 2007; Martin et al. 2010). We find no significant differences between the equivalent width measurements for the individual spectra of each exposure.

Table 2

PC3 index and adopted spectral type for our targets.

thumbnail Fig. 2

Zoomed-in view of the NOT/ALFOSC (left panel) and GTC/OSIRIS (right panel) co-added spectra of J-PLUS0807 and J-PLUS0903. The grey dashed lines mark Ca II H and K, Hδ, Hγ, Hβ, and Hα emission lines.
Table 3

Details of two-minute-cadence TESS LCs used in this work.

3.2 Light curve analysis

We queried the Mikulski Archive for Space Telescopes (MAST3) to fetch high-cadence photometric data for our sample. We found two-minute-cadence TESS-calibrated LCs for the two closest stars, J-PLUS0807 and J-PLUS0903, with TESS Input Catalog (TIC) IDs 461654150 and 166597074, respectively. Table 3 shows the details of the retrieved LCs, which are processed using the pipeline developed by the Science Processing Operations Centre (SPOC; Jenkins et al. 2016). The contamination ratio, Rcont, listed in the TIC (Paegert et al. 2022) is 10% and 0.22% for J-PLUS0807 and J-PLUS0903, respectively. To further study a possible contamination of the TESS photometry for these two stars, we used the tpfplotter4 (Aller et al. 2020) tool to explore the target pixel files (TPFs) of the fields of our targets. Thus, we only found a ∼ 1% contamination, obtained from the difference in Gaia magnitudes, from Gaia sources within the photometric apertures selected by the SPOC pipeline to process the LCs of the two stars.

We identified clear evidence of flaring activity and periodic variability in the retrieved two-minute-cadence LCs for J-PLUS0807 and J-PLUS0903, which are analysed in detail in Sects. 3.2.1 and 3.2.2. For the remaining five stars in our sample, which do not have processed, short-cadence TESS data, we used the Python package lightkurve (Lightkurve Collaboration 2018) to manually extract LCs from the TESS full frame images cutouts, but did not find any flare events or periodic variability signals. We also searched for time-resolved UV data from the NASA Galaxy Evolution Explorer (GALEX; Martin et al. 2005) mission for our targets, using the gPhoton (Million et al. 2016) database and software, but we did not find any.

thumbnail Fig. 3

LCs of J-PLUS0903 (sector 21; top-left panel) and J-PLUS0807 (sector 44; bottom-left panel). The right panel shows the largest flare event of the J-PLUS0903 LC.

3.2.1 Flares

For our analysis, we used the Pre-search Data Conditioning Simple Aperture Photometry (PDCSAP; Smith et al. 2012; Stumpe et al. 2012, 2014) flux, available in the TESS LCs retrieved for J-PLUS0807 and J-PLUS0903, which is already corrected from long-term trends, instrumental effects, and excess flux due to star-field crowding. We removed all data points with nonzero quality flags, after visually verifying that points with 512 ('impulsive outlier removed before co-trending') or 1024 ('cosmic ray detected on collateral pixel row or column') quality flags were not actually part of a real flare. We identified several flare events in all the J-PLUS0807 and J-PLUS0903 TESS LCs. For example, Fig. 3 shows the LC of J-PLUS0903 (top-left panel) and one of the LCs of J-PLUS807 (sector 44; bottom-left panel), with multiple flaring episodes observed in both of them. Moreover, the right panel provides a zoomed-in view of the flare event occurring around day 1890 (BJD 2457000 days) in the J-PLUS0903 LC.

We used the open-source Python software AltaiPony5 (Davenport 2016; Ilin et al. 2021) to automatically identify and characterise flares in the LCs. Prior to flare detection, we detrended the LCs using a Savitzky-Golay filter (Savitzky & Golay 1964) to remove rotational modulation trends. For flare detection, we followed the same procedure as Davenport et al. (2014), Doyle et al. (2019), Doyle et al. (2022), and Kumbhakar et al. (2023), identifying flares as two or more consecutive points that are 2.5 σ above the local scatter of the data (Chang et al. 2015). As also reported by Vida et al. (2017) and Doyle et al. (2018), we find no obvious relationship between flaring activity and rotational phase. AltaiPony automatically determines several flare properties, such as start and end times, flare amplitude, and equivalent duration, which is the area under the flare LC in units of seconds. Using the observed NOT/ALFOSC spectra and the tool Specphot6 (Rodrigo et al. 2024), developed and maintained by the Spanish Virtual Observatory7, we obtained the star quiescent flux in the TESS bandpass. We relied on the calculated flux and Gaia distances of our targets to derive the quiescent stellar luminosity and multiplied it by the equivalent duration to obtain the flare energy in the TESS bandpass. We obtained LTESS = 2.3 × 1029erg s–1 and LTESS = 2.2 × 1030 erg s−1 for the quiescent luminosity in the TESS bandpass of J-PLUS0807 and J-PLUS0903, respectively. Table 4 details the flare properties for each target.

The flare energy and rate obtained for our targets are typical of active, fast rotating mid- and late-M dwarfs (Doyle et al. 2019; Ramsay et al. 2020; Stelzer et al. 2022). With the observed flares for J-PLUS0807, we built the cumulative flare frequency distribution (FFD) to study the flare rate as a function of flare energy. This was not possible for J-PLUS0903 due to the low number of events available. FFDs can be expressed as a power-law relation (Stelzer et al. 2007; Lin et al. 2019): dvdEF∼EF−α,$\frac{d v}{d E_{\mathrm{F}}} \sim E_{\mathrm{F}}^{-\alpha},$(2)

where v is the cumulative flare rate for a given flare energy EF, and 1–α is the slope of a linear fit to a log-log representation. To fit the FFD, we relied on AltaiPony’s FFD.fit_powerlaw() method, which fits the power-law parameters simultaneously using the Markov chain Monte Carlo method described in Wheatland (2004). Since the detection probability decreases in the low-energy regime, where flares can go undetected due to the noise present in the LC, we discarded the low-energy tail of the FFD in the fit of the power-law (Hawley et al. 2014; Chang et al. 2015; Ilin et al. 2021). Figure 4 shows how the power-law relation breaks down around EF = 1031.5 erg, which is the threshold we applied to consider flares in the FFD fitting. Following this methodology, we obtained α = 1.74−0.17+0.20 for J-PLUS0807, which is in agreement with what Lin et al. (2019), Raetz et al. (2020), and Murray et al. (2022) found for their samples of 548, 56, and 85 flaring M dwarfs, respectively. We found that the less energetic flares, which are more frequent as illustrated in the FFD, are also shorter in duration and the easiest to detect with the J-PLUS observation strategy (see Sect. 2.1).

Table 4

Detailed flare properties of our targets with processed TESS LCs.

thumbnail Fig. 4

Cumulative FFD for J-PLUS0807. The solid red line represents the power-law fit obtained with AltaiPony.

3.2.2 Rotation periods

All TESS LCs retrieved for J-PLUS0807 and J-PLUS0903 show a clear periodic variability, which usually arises due to co-rotating star spots that appear and disappear from the line of sight. Therefore, we relied on a Lomb-Scargle periodogram (Lomb 1976; Scargle 1982), using the astropy Python package (Astropy Collaboration 2022), to search for the rotation period of each of our targets. Figure 5 shows the periodogram for each of the targets and the phase-folded LCs with the chosen periods, which are very prominent in the periodograms.

As a measure of the uncertainty of the peak position, we used the standard deviation of all the periods with a power greater than the half height of the periodogram peak. For J-PLUS0807, we computed the period using the data from all available sectors and obtained Prot = 0.3450 ± 0.0002 d, which is consistent with the values reported by Li et al. (2024) using TESS data from sectors 20, 45, 46 and 47, Seli et al. (2021) using only TESS data from sector 20, and Newton et al. (2016), who relied on photometry from the MEarth Project (Berta et al. 2012). For JPLUS0903, we obtained Prot = 1.69 ± 0.02 d, which is the first estimation for the rotation period of this object. We confirmed that our targets were the sources of the detected variability using the TESS_localize8 (Higgins & Bell 2023) Python package.

The computed rotation periods place our two targets as very fast rotators (Irwin et al. 2011), which is deeply interlinked with the activity level observed. After reaching the main sequence, low-mass stars slowly spin-down due to the loss of angular momentum by stellar winds, and thus their magnetic activity decreases over time (Yang et al. 2017; Davenport et al. 2019; Raetz et al. 2020); this decrease may also be dependent on the stellar metallicity (See et al. 2024), which makes obtaining robust age estimations for low-mass stars notoriously difficult. To explore this, we relied on stardate9 (Angus et al. 2019), a Python tool that combines isochrone fitting with gyrochronology for measuring stellar ages. In our case, we included magnitudes from the Two-Micron All Sky Survey (2MASS; Skrutskie et al. 2006), parallax and magnitudes from Gaia DR3, magnitudes from the Sloan Digital Sky Survey (SDSS; York et al. 2000), and the rotation periods obtained in this work as input parameters. Following this procedure, we obtained an age of 0.79−0.09+0.62 Gyr and 1.94−1.26+1.74 Gyr for J-PLUS0807 and J-PLUS0903, respectively, which is in agreement with the values found in the literature for fast rotators (Newton et al. 2016; Doyle et al. 2019). Here, the chosen value and uncertainties correspond to the median and ± 1 σ thresholds of the Markov chain Monte Carlo samples computed by stardate.

4 Planetary habitability

Understanding the impact of the magnetic activity of M dwarfs on a planet’s evolution and habitability is of crucial interest in the search for Earth-like planets. The common flaring activity and coronal mass ejections, together with the nearby habitable zone of these stars, can lead to substantial alteration of planetary atmospheres or even their erosion. It is unclear whether stellar flares are beneficial or detrimental to the habitability of exoplanets. It is possible that UV radiation emitted during flare events can trigger the development of prebiotic chemistry (Rimmer et al. 2018; Airapetian et al. 2020). Although abiogenesis would potentially be slower compared to prebiotic Earth due to the lower emission of M dwarfs at these wavelengths (Rugheimer et al. 2015; Ranjan et al. 2017), flares could provide the lacking UV energy (Buccino et al. 2007; Jackman et al. 2023). In this line, the flare events in Ca II H and K emission lines revealed in this work may play an important role.

Continued exposure to Ebol>1034 erg flares would make the presence of ozone layers impossible on any habitable zone terrestrial exoplanet orbiting an M dwarf (Tilley et al. 2019; Chen et al. 2021). Moreover, Berger et al. (2024) recently demonstrated that the 9000 K blackbody commonly assumed for flares underestimated the far-UV emission for 98% of their sample, which would significantly increase the number of stars with sufficient flaring activity to fall into the ozone depletion zone from previous studies. Following the relation provided by Seli et al. (2021), we converted the TESS energies of the detected flares to bolometric flare energies. Thus, we obtained a rate of 0.02 day−1 for Ebol > 1034 erg flares for J-PLUS0807, which is an order of magnitude lower than the rate found by Tilley et al. (2019) for the ozone layer to be eroded in habitable zone terrestrial exoplanets around M dwarfs. For J-PLUS0903, none of the flares exceeded this energy threshold.

thumbnail Fig. 5

TESS LCs (top panels), phase-folded LCs (bottom-left panels), and periodograms (bottom-right panels) for our two targets. The top panels show only a small section of the LCs for better visibility. Two different bin sizes are shown for the binned phase-folded LCs, with grey and black dots. In the periodograms, the dashed red line represents the 1% false alarm probability level.

5 Conclusions

This work serves as a follow-up study of the sample of M dwarfs identified in our previous work (Mas-Buitrago et al. 2022) with strong excess emission in the J-PLUS filters corresponding to Ca II H and K and Hα emission lines. Using low-resolution spectra collected with NOT/ALFOSC and GTC/OSIRIS, we measured the PC3 spectral index of our targets and spectroscopically confirmed the mid-M dwarf nature of six of them for the first time. We confirm that the strong excess emission detected in the J-PLUS photometry is caused by transient flare events, suggesting that common M dwarfs experience two types of flares, those already well known in Hα and those in Ca II H and K presented in this work. Work dedicated to the study of flares in large M dwarf samples usually focuses only on Hα flare events, which could lead to an underestimation of the number of flaring M dwarfs. In the future, multi-wavelength simultaneous observations will be essential for further studying the flaring activity of M dwarfs.

We analysed two-minute-cadence TESS LCs for JPLUS0807 and J-PLUS0903 and performed a thorough characterisation of the multiple flare events observed in them. We estimated the flare energies in the TESS bandpass and found them to be in the range 7.4 × 1030 −2.2 × 1033 erg. We find clear signs of a periodic variability in the TESS LCs, confirming the previously reported ultra-fast rotating nature of J-PLUS0807 with data from sectors 20, 44, 45, 46, and 47. Also, we computed the rotation period of for J-PLUS0903 for the first time, Prot = 1.69 ± 0.02 d.

This work demonstrates the potential of multi-filter photometric surveys such as J-PLUS or the upcoming J-PAS to systematically detect flare events in M dwarfs, especially episodes of strong Ca II H and K line emission, which may have important implications for exoplanetary space weather and habitability studies. Using a detection algorithm such as the one developed in Mas-Buitrago et al. (2022), it is possible to identify a sample of candidates that can be confirmed and analysed with spectroscopic follow-up and high-cadence photometric LCs from TESS or similar missions, such as K2. It also highlights the fundamental role of stellar flares in shaping the habitability of exoplanets. A high frequency of energetic flares implies that planets around these stars may experience significant atmospheric erosion and elevated levels of surface radiation, although it could also trigger the development of prebiotic chemistry.

Data availability

All the resources, including files with the reduced spectra and processed TESS LCs, and the code to reproduce the figures displayed in Sect. 3 are publicly available at GitHub10.

Acknowledgements

We acknowledge financial support from the Agencia Estatal de Investigación (AEI/10.13039/501100011033) of the Ministerio de Ciencia e Innovación and the European Union (ERC, SUBSTELLAR, project number 101054354) through projects PID2020-112949GB-I00 (Spanish Virtual Observatory https://svo.cab.inta-csic.es) and PID2019-109522GB-C5[3], and the Instituto Nacional de Técnica Aeroespacial through grant PRE-OVE. Based on observations made with the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma. This work is (partly) based on data obtained with the instrument OSIRIS, built by a Consortium led by the Instituto de Astrofísica de Canarias in collaboration with the Instituto de Astronomía of the Universidad Autónoma de México. OSIRIS was funded by GRANTECAN and the National Plan of Astronomy and Astrophysics of the Spanish Government. Based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. The data presented here were obtained [in part] with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOT. We thank NOT support astronomers David Jones and Tapio Pursimo for their help on site. This publication made use of VOSA, developed under the Spanish Virtual Observatory (https://svo.cab.inta-csic.es) project funded by MCIN/AEI/10.13039/501100011033/through grantPID2020-112949GB-I00. This work made use of tpfplotter by J. Lillo-Box (publicly available in www.github.com/jlillo/tpfplotter). We made extensive use of Python throughout the entire process, including the packages pandas (https://github.com/pandas-dev/pandas, numpy (Harris et al. 2020), matplotlib (Hunter 2007), astropy (Astropy Collaboration 2022), and lightkurve, a Python package for Kepler and TESS data analysis (Lightkurve Collaboration 2018).

Appendix A Additional figures

In this appendix we provide the J-PLUS SEDs of each target star, as discussed in Sect. 3.1. Figures A.1, A.2, A.3, A.4, A.5, A.6, and A.7 show the SED of J-PLUS0114, J-PLUS0226, J-PLUS0708, J-PLUS0744, J-PLUS0807, J-PLUS0903, and J-PLUS0914, respectively.

thumbnail Fig. A.1

J-PLUS photometry for J-PLUS0114.
thumbnail Fig. A.2

J-PLUS photometry for J-PLUS0226.
thumbnail Fig. A.3

J-PLUS photometry for J-PLUS0708.
thumbnail Fig. A.4

J-PLUS photometry for J-PLUS0744.
thumbnail Fig. A.5

J-PLUS photometry for J-PLUS0807.
thumbnail Fig. A.6

J-PLUS photometry for J-PLUS0903.
thumbnail Fig. A.7

J-PLUS photometry for J-PLUS0914.

Appendix B Record of observations

Table B.1

Low-resolution spectra collected with NOT/ALFOSC and GTC/OSIRIS.

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All Tables

Table 1

Targets selected for spectroscopic observation.

In the text
Table 2

PC3 index and adopted spectral type for our targets.

In the text
Table 3

Details of two-minute-cadence TESS LCs used in this work.

In the text
Table 4

Detailed flare properties of our targets with processed TESS LCs.

In the text
Table B.1

Low-resolution spectra collected with NOT/ALFOSC and GTC/OSIRIS.

In the text

All Figures

thumbnail Fig. 1

Co-added spectra observed with NOT/ALFOSC (left panel) and GTC/OSIRIS (right panel), sorted from top to bottom by the derived PC3 index (see Table 2). The grey shaded bands in the left panel show the spectral regions used to derive the PC3 index.
In the text
thumbnail Fig. 2

Zoomed-in view of the NOT/ALFOSC (left panel) and GTC/OSIRIS (right panel) co-added spectra of J-PLUS0807 and J-PLUS0903. The grey dashed lines mark Ca II H and K, Hδ, Hγ, Hβ, and Hα emission lines.
In the text
thumbnail Fig. 3

LCs of J-PLUS0903 (sector 21; top-left panel) and J-PLUS0807 (sector 44; bottom-left panel). The right panel shows the largest flare event of the J-PLUS0903 LC.
In the text
thumbnail Fig. 4

Cumulative FFD for J-PLUS0807. The solid red line represents the power-law fit obtained with AltaiPony.
In the text
thumbnail Fig. 5

TESS LCs (top panels), phase-folded LCs (bottom-left panels), and periodograms (bottom-right panels) for our two targets. The top panels show only a small section of the LCs for better visibility. Two different bin sizes are shown for the binned phase-folded LCs, with grey and black dots. In the periodograms, the dashed red line represents the 1% false alarm probability level.
In the text
thumbnail Fig. A.1

J-PLUS photometry for J-PLUS0114.
In the text
thumbnail Fig. A.2

J-PLUS photometry for J-PLUS0226.
In the text
thumbnail Fig. A.3

J-PLUS photometry for J-PLUS0708.
In the text
thumbnail Fig. A.4

J-PLUS photometry for J-PLUS0744.
In the text
thumbnail Fig. A.5

J-PLUS photometry for J-PLUS0807.
In the text
thumbnail Fig. A.6

J-PLUS photometry for J-PLUS0903.
In the text
thumbnail Fig. A.7

J-PLUS photometry for J-PLUS0914.
In the text

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